Why does the Canon ring mount for the 70-200 f4L USM cost $140 (e.g. here), whereas a third party mount costs as little as $20 (e.g. here)? Reading the comments on these two sites does not reveal anything seriously wrong with the Opteka mount. I can't understand the Canon costing seven times as much. Am I missing something?

To make money. That's pretty much it! Canon's reasonably priced and vast lens selection is a nice perk for Canon shooters, but the hidden fine-print where Canon makes a lot of money are in trivial little bits like collars and hoods, that are often times exorbitantly priced - and often times aren't included in the lens purchase as they are with other manufacturers. Everyone's gotta make a buck where they can.

Whether a third-party one is as good, or as reliable, depends on the third party - in general they're probably fine, but I'd inspect it before relying on it, to make sure it isn't too cheaply made - like with screw grooves carved into pot metal, or cheap hinges that could easily snap, or tripod screws that are too short and don't bind in well enough. If it passes basic tests like that, I'd personally be OK with a third party part.

$20 is insignificant money and worth a gamble on the 3rd party. Without having seen both in the flesh it is always hard to compare, but I would not be surprised if there is a build quality difference.

An example of that are lens hoods, since that has also been bought up. The Canon ones are much nicer than the ones I've seen from Sony/Sigma/Tamron/Olympus. In particular they put dark black fibres on the inside to prevent reflections. Of course you can argue if that's significant or even necessary, but it is a rarely mentioned extra feature.

And the cross brand value can be misleading. Show me the price for a comparable Sony or Nikon 70-400 f/4 with tripod mount for example